Sodium vapor lamps having improved starting means

ABSTRACT

The arc tube of a sodium vapor arc discharge lamp has an external electrical heater spaced from but in heat transfer relationship therewith. The heater is made of tungsten wire and is coiled around an insulating supporting rod.

ilnitefi States Patent [191 [111 5 Audesse Aug. 28, 1973 1 SODIUM VAPOR LAMPS HAVING IMPROVED STARTING MEANS [75] Inventor: Emery G. Audesse, Salem, Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

[22] Filed: June 2, 1972 [21] Appl. No.: 259,085

[52] US. Cl 315/47, 313/15, 315/46 [51] lint. C1. H0lj 7/24 [58] Field of Search 315/46, 47, 48; 313/15,17, 44

[56] References Cited UNITED STATES PATENTS 2,765,416 10/1956 Beese et a1" 313/15 Primary Examiner-Roy Lake Assistant Exg rinerDarwin R. l-lostetter Att5 rh e y -Norman l OMalley and James Theodosopoulos [57] ABSTRACT The are tube of a sodium vapor arc discharge lamp has an external electrical heater spaced from but in heat transfer relationship therewith. The heater is made of tungsten wire and is coiled around an insulating supporting rod.

6 Claims, 1 Drawing Figure BACKGROUND OF THE INVENTION 1. Field Of The Invention This invention relates to are discharge lamps and, in particular, to high pressure sodium vapor lamps.

2. Description Of The Prior Art Within the past few years, high pressure sodium vapor lamps have become commercially useful, especially for outdoor lighting applications, because of their high efficiency, generally in excess of 100 lumens per watt. The sodium operating vapor pressure in such lamps can vary from several millimeters to about 1,000 millimeters Hg.

Such lamps are called high pressure in order to distinguish them from low pressure sodium vapor lamps in which the sodium operating vapor pressure is in the order of a few microns. Low pressure sodium lamps have been in use for about 30 or 40 years, but, although efficient, they produce an unattractive monochromatic yellow light. The color of light from high pressure sodium lamps is considerably improved over that from low pressure sodium lamps.

High pressure sodium lamps generally comprise an alumina ceramic arc tube and an arc tube fill of sodium, mercury and an inert gas. Examples of such lamps are shown in the following U. S. Patent Nos. 3,248,590; 3,384,798; 3,448,319; 3,453,477; 3,485,343; 3,519,406; 3,521,108; 3,558,963.

One of the problems of high pressure sodium are discharge lamps relates to the starting thereof. Such lamps require a considerably higher starting voltage to initiate an arc discharge than do other types of arc discharge lamps, such as fluorescent, mercury or metal halide. This higher starting voltage requirement necessitates the use of a special ballast for high pressure sodium lamps.

SUMMARY OF THE INVENTION It is an object of this invention to provide a high pressure sodium vapor lamp having a reduced starting voltage, thereby eliminating the high voltage requirements of the ballast. Thus, a simpler more economical ballasting arrangement becomes quite practicable and, in fact, conventional mercury lamp ballasts can often be used. Although such a lamp is shown in copending application Ser. No. 214,000, filed l2-307l, entitled Arc Discharge Tube With Surrounding Starting Coil and assigned to the same assignee as the instant application, the invention disclosed in the instant application differs from that in said copending application.

A high pressure sodium vapor lamp in accordance with this invention comprises an arc tube having electrodes at its ends and containing a fill including sodium,- mercury and an inert gas. Adjacent the arc tube is a coiled tungsten heater mounted on an insulating support rod which is substantially parallel to the arc tube. The wattage and spacing of the heater are such that the heater will heat up the arc tube sufficiently in less than about 3 or 4 minutes to permit ignition thereof by a relatively low voltage between the electrodes. The relatively low voltage, about 220 volts for an existing 400 watt design is in relation to the approximate 2 kilovolt pulse that is normally needed to initiate the arc discharge in a high pressure sodium lamp and which high voltage pulse requires a special ballast.

The insulating rod is made of high temperature material, such as ceramic, because of its close proximity to the arc tube. Also, the diameter of the insulating rod should be less than about 40 percent the diameter of the arc tube in order that the amount of light intercepted and blocked by the insulating rod be small in relation to the total light output from the arc tube.

The tungsten heater is electrically connected to a thermal switch that opens and removes the heater from the circuit after the arc has been struck between the electrodes.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE in the drawing is an elevational view, partly broken away, of a high pressure sodium vapor lamp in accordance with this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A high pressure sodium vapor lamp in accordance with this invention comprises an outer glass envelope 1 which can be of elongated ovoid shape, such as is commonly used in high pressure sodium lamps, or of bulbous shape, such as is commonly used in mercury vapor and metal halide lamps. The neck of the envelope is closed by a re-entrant stem 2 having a press 3 through which extend stiff lead-in wires 4, 5 connected at their outer ends to a threaded shell 6 and center contact 7 of a conventional screw base.

Disposed within envelope 1 is an alumina arc tube 8 sealed at its lower end by niobium end cap 9 and at its upper end by niobium end cap l0.-Niobium tubes l1, 12 are brazed or welded to end caps 9, 10 and are used to support arc tube 8 as well as to conduct current to electrodes 13, 14 within the arc tube. In addition, one of the niobium tubes serves as an exhaust tube during manufacture and is used to introduce a fill including an inert gas (e.g. argon or xenon), sodium and mercury into the arc tube, after which the niobium tube is sealed, such as by a cold weld.

Arc tube 8 is supported within envelope 1 by a structure consisting of side support wire 15, vertical support wire 16, metal strap 17 and horizontal metal strap 18. Support wire 15 is welded to lead-in wire 5, support wire 16 is welded to wire 15, strap 17 is welded to wire 16, strap 18 is welded to strap 17 and strap 18 is connected to upper niobium tube 12. Electrical connection between lead-in wire 5 and upper electrode 14 is established by said structure.

The lower end of arc tube 8 is supported by metal strap 19 which is connected between niobium tube 11 and support wire 27, with support wire 27 being welded to lead-in wire 4. Electrical connection is also established thereby between lead-in wire 4 and lower electrode l3.

Disposed parallel to are tube 8, and spaced about l/l6 inch therefrom is a coiled coil heater 20 which is made of a length of 4 mil tungsten wire primary coiled on an 8 mil mandrel and then secondary coiled on an alumina rod 21 having a diameter of 3/32 inch. The ends of rod 21 are supported by straps 18, 19. The lower end of heater 20 is electrically connected directly to strap 19. However, in order to permit removal of heater 20 from the electrical circuit after an are discharge has been struck between electrodes 13 and 14, electrical connection to the upper end of heater 20 is established through a bimetallic switch 22. For this purpose, and also for increased support of rod 21, the upper end of heater 20 is connected to a metal strap 23 which tightly encircles rod 21 and which is connected to a vertical metal post 24. Post 24 is insulatively supported on side wire by means of glass rod 25. One end of switch 22 makes electrical contact with post 24 and the other end of switch 22 is welded to vertical wire 16.

Attached to the upper and lower ends of side wire 15 are spring fingers 26 which press against the interior wall of envelope 1 and provide increased support for the arc tube.

In operation the lamp was used in a fixture for a mer cury vapor lamp which utilized a reactor ballast, the output voltage of which was 216 volts. At this voltage, heater drew 190 watts. Within 3 minutes, are tube 8 had been heated sufficiently by heater 20 so that an arc was struck between electrodes 13 and 14 as a result of the 216 volts thereacross. Shortly thereafter, switch 22 reached its energization temperature and opened, thereby removing heater 20 from the circuit. The efficiency of this lamp was 105 lumens per watt.

in this example are tube 8 had a diameter of 0.350 inches and rod 21 had a diameter of 0.093 inches. The amount of light lost as a result of interception by rod 21 and heater 20 was less than 5 percent of the total light emitted by are tube 8. in order that this amount of lost light be kept less than about 5 percent of the total, the diameter of rod 21 should be less than about 40 percent of the diameter of arc tube 8.

I claim:

1. A high pressure sodium vapor lamp comprising: a glass envelope; an alumina arc tube disposed within said envelope and having electrodes sealed therein and containing a filling including sodium, mercury and an inert gas; a heater disposed parallel to said are tube and closely spaced therefrom, said heater comprising tung sten wire coiled around an insulating rod; and a thermal switch disposed within said heater; said heater being effective to raise the temperature of the arc tube to at least approximately C, thereby substantially reducing the voltage required to start an are between the electrodes, said heater becoming electrically disconnected upon starting of an are between said electrodes.

2. The lamp of claim 1 wherein said heater comprises a coiled coil.

3. The lamp of claim 1 wherein said insulating rod is made of alumina.

4. The lamp of claim 1 wherein the diameter of said insulating rod is less than about 40 percent of the diameter of said are tube.

5. The lamp of claim 1 wherein said switch is normally closed and where the opening temperature of said switch is such that upon electrical energization of said lamp, said switch opens in about 3 or 4 minutes.

6. The lamp of claim 1 wherein the construction and spacing of said heater is such that, upon normal operation of said lamp, said heater heats said are tube sufficiently that a starting voltage of less than about 400 volts ignites an arc discharge between the electrodes of said are tube.

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2. The lamp of claim 1 wherein said heater comprises a coiled coil.
 3. The lamp of claim 1 wherein said insulating rod is made of alumina.
 4. The lamp of claim 1 wherein the diameter of said insulating rod is less than about 40 percent of the diameter of said arc tube.
 5. The lamp of claim 1 wherein said switch is normally closed and where the opening temperature of said switch is such that upon electrical energization of said lamp, said switch opens in about 3 or 4 minutes.
 6. The lamp of claim 1 wherein the construction and spacing of said heater is such that, upon normal operation of said lamp, said heater heats said arc tube sufficiently that a starting voltage of less than about 400 volts ignites an arc discharge between the electrodes of said arc tube. 